Lecture Electromechanical energy conversion: DC Machines presents the following content: Commutator action, effect of armature MMF, analytical fundamentals: Electric – Circuit aspects, analytical fundamentals: magnetic – circuit aspects, analysis of steady – state performance, permanent – magnet DC machines, commutation and interpoles, serial universal motors.
Nguyễn Công Phương ELECTROMECHANICAL ENERGY CONVERSION DC Machines Contents I Magnetic Circuits and Magnetic Materials II Electromechanical Energy Conversion Principles III Introduction to Rotating Machines IV Synchronous Machines V Polyphase Induction Machines VI DC Machines VII.Variable – Reluctance Machines and Stepping Motors VIII.Single and Two – Phase Motors IX Speed and Torque Control sites.google.com/site/ncpdhbkhn DC Machines Introduction Commutator Action Effect of Armature MMF Analytical Fundamentals: Electric – Circuit Aspects Analytical Fundamentals: Magnetic – Circuit Aspects Analysis of Steady – State Performance Permanent – Magnet DC Machines Commutation and Interpoles Compensating Windings 10 Serial Universal Motors sites.google.com/site/ncpdhbkhn Introduction (1) • Can be designed to display a wide variety of volt – ampere or speed – torque characteristics • Frequently used in applications requiring a wide range of motor speeds or precise control of motor output sites.google.com/site/ncpdhbkhn http://www.electrical4u.com/permanent‐ magnet‐dc‐motor‐or‐pmdc‐motor/ Introduction (2) Tmech poles 2 r 90o Fa1 Tmech poles 2 2 2 d Fa1 Ca ia 2m poles poles Ca d ia K a d ia 2 m sites.google.com/site/ncpdhbkhn Tmech Direct axis d Fa1 sin r Quadrature axis Brushes Field coil Armature coils Field Armature Introduction (3) Quadrature axis 1.5 Voltage Rectified coil voltage Rectified coil voltage Brush voltage 100 200 Tmech 300 Time 400 500 600 poles Ca d ia K a d ia 2 m 0.5 Direct axis Brushes Field coil Armature coils ea Tmechm poles ea Ca d m K a d m 2 m sites.google.com/site/ncpdhbkhn Introduction (4) d Quadrature axis d Pd N f i f Tmech Direct axis Brushes Field coil Armature coils N f if poles Ca d ia K a d ia ; ea K a d m Tmechm 2 m ea K a d m ea sites.google.com/site/ncpdhbkhn m n ea ea n0 m Introduction (5) Field Armature To dc source Separately – excited Series field Series Series field Shunt Compound sites.google.com/site/ncpdhbkhn Introduction (6) • Generators: – The required field current is a very small fraction of the rated armature current; on the order of to percent in the average generator – A small amount of power in the field circuit may control a relatively large amount of power in the armature circuit, i.e., the generator is a power amplifier – Often used in feedback control systems when control of the armature voltage over a wide range is required – The terminal voltage decreases slightly with an increase in the load current • Field Armature To dc source Separately – excited Motors: – The field flux is nearly constant sites.google.com/site/ncpdhbkhn Introduction (7) • The field current is the same as the load current, so that the air – gap flux & hence the voltage vary widely with load are not often used sites.google.com/site/ncpdhbkhn Series field Series generator 10 DC Machines Introduction Commutator Action Effect of Armature MMF Analytical Fundamentals: Electric – Circuit Aspects Analytical Fundamentals: Magnetic – Circuit Aspects Analysis of Steady – State Performance Permanent – Magnet DC Machines Commutation and Interpoles Compensating Windings 10 Serial Universal Motors sites.google.com/site/ncpdhbkhn 44 Permanent – Magnet DC Machines (1) http://www.infolytica.com/en/applications/ex0190/ • • • • The principal difference between permanent – magnet dc machines & those discussed previously is that they have a fixed source of field – winding flux which is supplied by a permanent magnet Widely found in a wide variety of low-power applications The field winding is replaced by a permanent magnet Advantages: – Simple construction – Do not require external excitation & power to create magnetic fields – Maybe smaller & cheaper • Disadvantages: – The risk of demagnetization due to excessive currents in the motor windings or due to overheating of the magnet – Permanent magnets are somewhat limited in the magnitude of air – gap flux density that they can produce sites.google.com/site/ncpdhbkhn 45 Ex Permanent – Magnet DC Machines (2) Both the rotor and the outer shell are made of infinitely permeable magnetic material (μ → ∞) The magnet is made of neodymium-iron-boron tg = 0.5mm, tm = 3.5mmm Ignoring the effects of rotor slots, find the magnetic flux density in the air gap Bm 0 tm tg tm 3.5 H m 4 107 H m 28 107 H m tg 0.5 http://www.infolytica.com/en/applications/ex0190/ Bm Bg 1.09T 2t m 2t g sites.google.com/site/ncpdhbkhn 46 Permanent – Magnet DC Machines (3) Ia Ea K mm Vt Ea K a d m ; Tmech Ea I a m ; sites.google.com/site/ncpdhbkhn Km Ka d 47 Ex Permanent – Magnet DC Machines (4) A permanent – magnet dc motor has an armature resistance of 1.03Ω When operated at no load from a dc source of 50V, it is observed to operate at a speed of 2100 rpm & to draw a current of 1.25A Find: a) The torque constant Km? b) The no – load rotational losses of the motor? c) The power output of the motor when it is operating at 1700 rpm from a 48-V source Ia Vt Ea Vt Ea Ra I a Ea Vt Ra I a 50 1.03 1.25 48.7V 2100 2 m 220 rad/s 60 Ea 48.7 V Km 0.22 r/s m 220 Rotational losses Ea I a 48.7 1.25 61W sites.google.com/site/ncpdhbkhn 48 Ex Permanent – Magnet DC Machines (5) A permanent – magnet dc motor has an armature resistance of 1.03Ω When operated at no load from a dc source of 50V, it is observed to operate at a speed of 2100 rpm & to draw a current of 1.25A Find: a) The torque constant Km? b) The no – load rotational losses of the motor? c) The power output of the motor when it is operating at 1700 rpm from a 48-V source m Ia Vt Ea 1700 2 178 rad/s 60 Ea K mm 0.22 178 39.2V Vt Ea Ra I a I a Vt Ea 48 39.2 8.54A Ra 1.03 Pmech Ea I a 39.2 8.54 335W Pshaft Pmech rotational losses 335 61 274W sites.google.com/site/ncpdhbkhn 49 DC Machines Introduction Commutator Action Effect of Armature MMF Analytical Fundamentals: Electric – Circuit Aspects Analytical Fundamentals: Magnetic – Circuit Aspects Analysis of Steady – State Performance Permanent – Magnet DC Machines Commutation and Interpoles Compensating Windings 10 Serial Universal Motors sites.google.com/site/ncpdhbkhn 50 Commutation and Interpoles (1) • The most important: the ability to transfer the necessary armature current through the brush contact at the commutator without sparking & without excessive local losses & heating of the brushes & commutator • Sparking causes destructive blackening, pitting, and wear of both the commutator & the brushes, conditions which rapidly become worse & burn away the copper & carbon • Sparking may be caused by faulty mechanical conditions, such as chattering of the brushes or a rough, unevenly worn commutator sites.google.com/site/ncpdhbkhn 51 Commutation and Interpoles (2) • The attainment of good commutation is more an empirical art than a quantitative science • The principal obstacle to quantitative analysis lies in the electrical behavior of the carbon-copper (brush-commutator) contact film • The film’s resistance is nonlinear and is a function of current density, current direction, temperature, brush material, moisture, and atmosphere pressure • The film’s behavior in some respects is like that of an ionized gas or plasma sites.google.com/site/ncpdhbkhn 52 Commutation and Interpoles (3) sites.google.com/site/ncpdhbkhn 53 DC Machines Introduction Commutator Action Effect of Armature MMF Analytical Fundamentals: Electric – Circuit Aspects Analytical Fundamentals: Magnetic – Circuit Aspects Analysis of Steady – State Performance Permanent – Magnet DC Machines Commutation and Interpoles Compensating Windings 10 Serial Universal Motors sites.google.com/site/ncpdhbkhn 54 Compensating Windings (1) • For machines subjected to heavy overloads, rapidly changing loads, or operation with a weak main field, there is the possibility of trouble other than simply sparking at the brushes • At the instant when an armature coil is located at the peak of a badly distorted flux wave, the coil voltage may be high enough to break down the air between the adjacent segments to which the coil is connected and result in flashover, or arcing, between segments • The breakdown voltage here is not high, because the air near the commutator is in a condition favorable to breakdown, due to the presence of the plasma carrying the armature current between the brushes & the commutator sites.google.com/site/ncpdhbkhn 55 Compensating Windings (2) • Flashing between segments may quickly spread around the entire commutator and constitutes a direct short circuit on the line • Solution: compensate or neutralize the armature mmf under the pole faces • Disadvantage: expensive Commutating winding Shunt field Armature Field rheostat sites.google.com/site/ncpdhbkhn Series field Compensating winding 56 DC Machines Introduction Commutator Action Effect of Armature MMF Analytical Fundamentals: Electric – Circuit Aspects Analytical Fundamentals: Magnetic – Circuit Aspects Analysis of Steady – State Performance Permanent – Magnet DC Machines Commutation and Interpoles Compensating Windings 10 Serial Universal Motors sites.google.com/site/ncpdhbkhn 57 Serial Universal Motors • Series universal motor: the rotor & stator structures of a series connected motor are properly laminated to reduce ac-eddy current losses • It has the convenient ability to run on either ac or dc current & with similar characteristics • Universal motor: a single – phase series motor • Used where light weight is important (e.g., vacuum cleaner, kitchen appliances, portable tools) and usually operate at high speed (1500 to 15000 rpm) • Advantage: highest horsepower per dollar in the fractional – horsepower range • Disadvantages: noise, relative short life sites.google.com/site/ncpdhbkhn Series field Ea Ia Armature 58 Va ... Materials II Electromechanical Energy Conversion Principles III Introduction to Rotating Machines IV Synchronous Machines V Polyphase Induction Machines VI DC Machines VII.Variable – Reluctance Machines. .. Shunt field 23 Ex Analytical Fundamentals: Electric – Circuit Aspects (2) A 25-kW 110-V separately – excited DC machines is operated at a constant speed of 2400 rpm with a constant field current... I a Ia 24 Ex Analytical Fundamentals: Electric – Circuit Aspects (3) A 25-kW 110-V separately – excited DC machines is operated at a constant speed of 2400 rpm with a constant field current